v9.2.1: real filenames back in open-link URLs (was literal "file")

User report: downloaded/opened files named just "file" — v9.2 put a
literal file.docx/file.pdf at the end of open-link URLs (treated as
cosmetic since Content-Disposition carries the real name, but Word
names its URL-copy from the URL path and browsers fall back to the
URL filename)

- OpenLinkService.CreateLink takes fileName and appends it (URL-encoded,
  Thai-safe) — version: {base}_v{N}.docx/.pdf per FR-15, draft:
  {template}_{timestamp}.docx per FR-10 naming policy
- Draft route split into /d/{templateId}/{stamp}/{fileName} (rebuilds
  objectKey + HMAC-validates as before); version route unchanged shape
  with real name
- Tests: URL tails assert real names (คำร้อง_v1.pdf/_v1.docx, draft
  template name), no /file.docx; new tamper tests — modified stamp/
  templateId and swapped docx↔pdf extension (version route) → 404
- backend 89/89; QA reviewer PASS (1 round, minors 2 fixed)

Co-Authored-By: Claude Code <noreply@anthropic.com>

backend/WordVersioning.Api/Controllers/DocumentsController.cs

@@ -538,7 +538,11 @@ public class DocumentsController(
         if (objectKey is null)
             return NotFound("version นี้ไม่มีสำเนา PDF");
 
-        var url = openLinks.CreateLink($"v:{documentId}:{versionId}", isPdf ? "pdf" : "docx", out var expiresAt);
+        // ชื่อไฟล์ตามหลักเดิม — อยู่ท้าย URL (Word/browser ตั้งชื่อตาม) และใน Content-Disposition ตอน stream
+        var baseName = row.FileName.EndsWith(".docx", StringComparison.OrdinalIgnoreCase)
+            ? row.FileName[..^5] : row.FileName;
+        var fileName = $"{baseName}_v{row.VersionNumber}.{(isPdf ? "pdf" : "docx")}";
+        var url = openLinks.CreateLink($"v:{documentId}:{versionId}", isPdf ? "pdf" : "docx", fileName, out var expiresAt);
 
         logger.LogInformation(
             "Version download link issued documentId={DocumentId} versionId={VersionId} userId={UserId} expiresAt={ExpiresAt}",